There are Patent Documents 1 and 2. They relate to a capacitance-type apparatus for measuring a flow rate of a powder.
Generally, when a flow rate is measured, the temperature of an object used to measure a flow rate is changed based on the conditions, such as the temperature or humidity of the air (atmosphere) used as a carrier gas. Thus, it is necessary to correct without any delay any errors that may be caused by the change of the temperature of the object.
To solve this problem, Patent Document 1 discloses an invention relating to an apparatus for measuring a flow rate of a powder. It is constituted of:
an electrode (“a measuring electrode”) for measuring a capacitance of a powder that can detect a flow rate of a powder based on the change of a capacitance, and which is disposed at the pathway of the flow of the powder, and
an electrode (“an adjusting electrode”) for adjusting any errors of the measurements caused by the change of the environmental conditions near the measuring electrode while the air that is used as a carrier gas flows,
wherein the measuring electrode and the adjusting electrode are disposed in parallel. (See claim 1 of Patent Document 1.)
Namely, for the apparatus of Patent Document 1, two identical sensors for measuring a flow rate are disposed in parallel. One sensor is provided to a powder carried by air used as a carrier gas. The other sensor is provided to only air and is used for adjusting any errors of the measurements. Based on the difference between the outputs of the two sensors, the flow rate of the powder can be measured.
However, for the apparatus of Patent Document 1, it is necessary to provide air (as a carrier gas) with properties identical to both the measuring electrode and the adjusting electrode.
For example, when a flow rate of a powder carried by compressed air is measured, compressed air having identical properties must be provided to the adjusting electrode. The compressed air is not used for carrying the powder, and as a result, is uselessly exhausted into the atmosphere. Thus, twice the amount of compressed air is required to measure the flow rate of the powder. Further, when a flow rate of a powder carried by suctioned air is measured, suctioned air with identical properties must also be provided to the adjusting electrode. Thus, twice the amount of suctioned air is also required to measure the flow rate of the powder. For each case, the energy consumed for measuring the flow rate increases.
Patent Document 2 discloses an invention relating to an apparatus for measuring a flow rate of a powder that has a high temperature, ranging from 500-1200° C. (see claim 1 of Patent Document 2). It is constituted of an apparatus for measuring a flow rate of a powder passing through a pipe based on the change of the capacitance, the apparatus comprising:
a pipe for carrying a powder,
electrodes for measuring a flow rate comprising:                a pair of a source electrode and a sensing electrode disposed around the pipe, and facing each other, wherein the electrodes have a curved shape,        a guard electrode disposed between the source electrode and the sensing electrode,        
a support structure to hold the electrodes, wherein the structure is concentrically disposed around the pipe and has a tubular shape,
wherein the source electrode and the guard electrode are formed by a heat-resistant and conductive material disposed at the inner surface of the support structure with a spiral pattern.
Namely, for the invention of Patent Document 2, since the powder, having a high temperature (ranging from 500-1200° C.), for measuring its flow rate passes through the pipe, and since the apparatus has a configuration such that the pipe is disposed in the support structure having the electrodes, it can prevent the electrodes from separating from the support structure because of thermal influence.
However, since the powder having a high temperature flows in the pipe, the temperature in the pipe increases. Thus, there is a possibility that the capacitance of the object for measurement will change. Patent Document 2 does not disclose or suggest how the effect caused by the change of the capacitance should be adjusted. Thus, there is a problem in that the measured value of the flow rate changes with time.
There are Patent Documents 3 and 4, relating to an apparatus for measuring a flow rate of a powder that uses capacitance. These documents were filed by the same applicant as that of this application, and do not affect the patentability of this invention.    Patent Document 2: Japanese Patent Laid-open Publication No. 2001-21397    Patent Document 3: Japanese Patent Laid-open Publication No. 2007-121272    Patent Document 4: Japanese Patent Laid-open Publication No. 2006-329874